High optical power applications, such as high power fiber lasers, require strong Bragg gratings which are resistant to the high intensity of light circulating in the optical fiber.
Gratings obtained using the defect-resonant UV-induced physical process that is commonly used for the writing of fiber Bragg gratings (FBGs) in silica fibers are restricted to photosensitive fibers and cannot generally be inscribed in the rare-earth doped fibers used as laser gain media. This in turn implies that fiber laser cavities will require fusion splices between the active fiber and the photosensitive ones. Those splices may lead to additional intra-cavity losses and are not suited for some active fiber geometries, particularly when high power operation is required. Therefore, new approaches to the manufacture of FBGs need to be developed in order to inscribe grating directly into the active fiber.
In addition, the optical fibers used within or in conjunction with high power devices and Bragg gratings inscribed in these fibers need to withstand increasingly high optical power as the power output of such devices also increases. The amount of loss induced during the FBG writing process in any type of fiber will define its power handling, such loss can then be detrimental to some applications.
The refractive index change resulting from the nonlinear interaction of focused femtosecond pulses with glass seems a very promising alternative to the well-known defect-resonant UV-induced physical process. As shown in M. Bernier, D. Faucher, R. Vallee, A. Saliminia, G. Androz, Y. Sheng, and S. L. Chin, “Bragg gratings photoinduced in ZBLAN fibers by femtosecond pulses at 800 nm,” Opt. Lett. 32, 454-456 (2007), infrared femtosecond (fs) pulses with a first-order phase-mask can be used to write efficient FBGs in both doped and undoped fluoride fibers for operation at 1.5 μm. Alternatively, FBGs written with the scanning phase-mask technique using IR fs pulses also proved crucial to the development of silica fiber lasers doped with either erbium (see E. Wikszak, J. Thomas, J. Burghoff, B. Ortaç, J. Limpert, S. Nolte, U. Fuchs, and A. Tünnermann, “Erbium fiber laser based on intracore femtosecond-written fiber Bragg grating,” Opt. Lett. 31, 2390-2392 (2006)) as well as with ytterbium (see E. Wikszak, J. Thomas, S. Klingebiel, B. Ortag, J. Limpert, S. Nolte, U. Fuchs, and A. Tunnermann, “Linearly polarized ytterbium fiber laser based on intracore femtosecond-written fiber Bragg gratings,” Opt. Lett. 32, 2756-2758 (2007)) active ions. In the latter case, a maximum output power of 100 mW at 1040 nm was obtained from an ytterbium-doped panda-type fiber with a laser slope efficiency of 27%. The second-order FBGs involved in this experiment had a peak reflectivity of 65% and 45% for each polarization, respectively.
MIHAILOV et al. (U.S. Pat. Nos. 6,993,221 and 7,031,571) discuss the writing of Bragg gratings in optical fibers which are not photosensitive, using ultrashort pulses through a phase mask. They argue that contrary to prior art assertions, gratings can be written using femtosecond pulses of intensity high enough to generate a refractive index change in the fiber, while still being below the damage threshold of the phase mask. The disclosed technique allegedly alleviates the need for photosensitising the fiber and for post processing of the grating through annealing or the like. MIHAILOV et al. further prone the selection of a phase mask having a pitch selected to induce a high order Bragg resonance at the wavelength of interest, in order to limit the angular dispersion of the long wavelength writing beam induced by a lower order phase mask. However, for high power applications, the strength of the high order grating at the wavelength of interest may not always be sufficient.
There remains a need for a method of writing Bragg gratings particularly suitable for high power fiber lasers or similar applications which alleviates at least some of the drawbacks of the prior art.